Floatable offshore drilling rigs are widely used in the exploration and exploitation of hydrocarbon reservoirs under the sea floor, in particular at relatively high water depths.
One type of floatable drilling rig is the semi-submersible drilling rig that typically obtains its buoyancy from ballasted, watertight pontoons located below the ocean surface and wave action. The operating deck can be located high above the sea level due to the high stability of the design, and therefore the operating deck is kept well away from the waves. Structural columns connect the pontoons and operating deck. Another type of floatable drilling vessels is a drill ship.
Offshore drilling rigs and, in particular, floatable offshore drilling rigs utilise a variety of tubular equipment, such as drill pipes. Marine risers are a particular type of tubular equipment used in subsea drilling operations. The marine riser (in the present disclosure also simply referred to as the riser) is made up of marine riser joints. Most marine riser joints are large elongated structures, typically defining a main tube and a number of secondary tubes as well as a number of Buoyancy elements. During the drilling operation the marine riser joints are typically assembled to a string of marine riser joints forming the riser and lowered towards the sea floor so as to create a string of marine riser joints extending from the drilling rig to the sea floor. The drill string is then advanced through the central tube of the marine riser. Marine risers joints are very large and heavy tubular elements; typical risers joints are 50-75 ft and even 90 ft long and weigh many tons. Consequently they are difficult to handle, for example when loading or offloading them to/from the drilling rig, e.g. from/to a supply ship.
U.S. Pat. No. 4,129,221 discusses riser handling on a drill ship where the riser joints are stored in horizontal orientation and hoisted to the well centre by a crane.
However, on many drilling rigs it is preferred to store the riser joints in vertical orientation, as a vertical storage requires less deck space on the operational deck. U.S. Pat. No. 8,052,369, discloses an offshore drilling rig where tubulars are stored in a shaft in vertical position. The tubulars are lifted out of the shaft and across the main deck of the drilling rig towards a catwalk machine, where they are brought into horizontal position and axially fed through a V-door in the derrick to the well centre.
It is generally desirable to provide a drilling rig and corresponding riser handling apparatus allowing efficient, loading and offloading of riser joints onto/from the drill rig (e.g. for maintenance of the riser joints) and/or for efficient maintenance of riser joints.